1. Field of the Invention
The present invention relates to a backlight unit of direct emission type in which light sources are arranged at the rear side of the light emitting surface of the backlight unit. The present invention also relates to a LCD device including such a backlight unit.
2. Description of the Related Art
Backlight units are known in the art as planar light-emitting devices. The backlight units are generally classified into two types. The first type is a direct emission unit, in which a plurality of light sources (i.e., lamps) are arranged at the rear side of the light-emitting surface of the backlight unit. The second type is an edge-light emission unit, in which the light emitted by the light sources is guided toward the light-emitting surface by an optical guide plate. The direct emission unit has a larger light-emitting surface and thus can attain a higher luminance, compared to the backlight unit of the edge-light emission type. Having these advantages, the direct emission unit is suitable for use in, particularly, a large-screen LCD device.
FIG. 15 is an exploded perspective view illustrating the structure of a conventional backlight unit of the direct emission type. As shown in the drawing, the backlight unit 201 includes therein a reflecting plate 211, a plurality of lamps 212, a lamp-supporting base 215, a diffusion plate 216, an optical sheet 217, and a backlight chassis 218. The lamps 212 are connected at one end to an inverter 213, and at the other end to a return substrate 214a. A ground potential is applied to the return substrate 214a through a return cable 214b. Part of the light emitted from the lamps 212 is directly irradiated onto the diffusion plate 216, and other part of the light is reflected by the reflecting plate 211 and then irradiated onto the diffusion plate 216. The outer surface of the diffusion plate 216 generally defines the light-emitting surface of the backlight unit 201.
FIG. 16 is an exploded perspective view depicting a LCD device 200 that includes the backlight unit 201. In the LCD device, a liquid crystal panel 202 is connected to an X-direction drive circuit 204 and a Y-direction drive circuit 205 by respective TCPs (Tape Carrier Packages) 206. The panel 202 is clamped and held between a shield front frame 203 and the backlight unit 201. The liquid crystal panel 202 is mounted on the light-emitting surface of the backlight unit 201, or more precisely, on the optical sheet 217 thereof. The panel 202 controls the transmission of the backlight emitted by the backlight unit 210, to thereby display desired images.
In the backlight unit 201, the lamps 212 are interposed between the reflecting plate 211 and the diffusion plate 216. Therefore, the heat generated by the lamps 212 is hardly radiated outside. The heat generated by the lamps 212 heats the diffusion plate 216. Mounted on the diffusion plate 216, the liquid crystal panel 202 is heated by the heat it receives from outside, and also by the heat radiated from the diffusion plate 216. Consequently, in the LCD device 200, the liquid crystal panel 202 may be heated up to an undesired temperature, particularly in a higher-temperature ambient, and may fail to display images of desired quality. In other words, the LCD device 200 may have its rated operating temperature lowered by the backlight unit.
As in most cases, the lamps 212 exhibit a temperature characteristic. That is, the luminescence efficiency of the lamps 212 increases as the temperature rises, reaches a peak efficiency at a specific temperature, and decreases as the temperature falls from the specific temperature. Thus, in the backlight unit 201, there is a problem in that the lamps 212 have a reduced luminescence efficiency when their operating temperature rises above the specific temperature, due to the heat they generate. The backlight unit 201 has another problem in that when the lamps 212 are used at high temperatures, the lamps are degraded due to heat and thus the lifetime of the lamps 212 is reduced. As described heretofore, the backlight unit 201 has problems due to the heat that the lamps 212 itself radiate.
Various techniques of radiating the heat generated by the lamps 212 of the backlight unit are described in, for example, Jpn. Pat. Appln. Publication Nos. 2002-196326, 2003-84280, and 8 (1996)-29785. The technique described in Pat. Publication No. 2002-196326 is to radiate the heat of the lamps 212 through the ventilation holes that are formed in the reflecting plate 211 disposed at the rear side of the lamps 212. The technique described in Publication No. 2003-84280 is to radiate the heat of the lamps 212 from the reflecting plate 211, via a heat-radiating body that contacts the lamps 212 with the reflecting plate 211. The technique described in Publication No. 8-29785 is to provide a transparent substrate 220 with a stripe member 219 having a comparatively higher thermal conductivity, between the diffusion plate 216 and the lamps 212 as shown in FIG. 17, and to radiate the heat absorbed in the transparent substrate 220 through a heat-radiating member 221.
FIG. 18 is a sectional view taken parallel to the Y-Z plane in FIG. 16, and illustrates the LCD device 200 being assembled. It is desired in the LCD device 200 that the width (width D, in FIG. 18) of frame of the LCD device be reduced so that the LCD device 200 may become smaller without reducing the effective display area. To this end, the TCPs 206 are bent in the LCD device 200 to dispose the X-direction drive circuit 204 and the Y-direction drive circuit 205 on the rear side of the backlight unit 201 in the vicinity of the reflecting plate 211 of the backlight unit 201. In this configuration, however, if the heat is radiated from the reflecting plate 211 as is the case described in Publications Nos. 2002-196326 and 2003-84280, the X-direction drive circuit 204 and the Y-direction drive circuit 205 will be heated, thereby reducing the reliability of the drive circuits 204 and 205. If a transparent substrate having a stripe member is provided between the diffusion plate 216 and the lamps 212, as is the case described in Publication No. 8-29785, the luminance at the light-emitting surface will be reduced.
Another type of the backlight unit is also known. This type is called double-surface backlight unit, which has another light-emitting surface at the rear side thereof, in addition to the light-emitting surface on the front side. A double-surface backlight unit is described in, for example, Jpn. Pat. Appln. Publication No. 2000-338483. FIG. 19 is an exploded perspective view of the double-surface backlight unit 201a described therein. The double-surface backlight unit 201a includes a front part and a rear part, which are symmetrical with respect to the lamps 212. Each of the front and rear parts has a lamp-supporting base 215, a diffusion plate 216, an optical sheet 217, and a backlight chassis 218. The double-surface backlight unit 201a has no component that corresponds to the reflecting plate 211 (FIG. 15) that is opposed to the diffusion plate 216. The lamps 212, which are arranged in a row, emit light through both the light-emitting surfaces.
In the double-surface backlight unit 201a, the heat generated from the lamps 212 involves a problem as in the case of the single-surface backlight unit 201. Having no reflecting plate 211 at the rear side of the lamps 212, the double-surface backlight unit 201a cannot adopt a structure that radiates heat from the rear side, differently from the backlight units described in Publications Nos. 2002-196326 and 2003-84280. If the double-surface backlight unit 201a has a transparent substrate 220 provided between the diffusion plate 216 and the lamps 212 as is the case described in Publication No. 8-29785, the luminescence efficiency at the light-emitting surfaces will decrease similarly to the single-side backlight unit 210 described above.